1. Field of the Invention
This invention relates to a light beam scanner to scan a light beam, and more particularly to a light beam scanner in which a small mirror supported by a torsion spring (torsion bar) is vibrated to deflect a light beam.
2. Description of the Related Art
Recently, a light beam scanner to scan a light beam such as a laser beam is used for optical apparatuses such as a bar code reader, a laser printer, a head mount display and so on, or a light input device of an input unit of an infrared camera. One light beam scanner is proposed in which a small mirror is vibrated. The small mirror is formed using the well known silicon micromachining technique.
As such a light beam scanner, a light beam scanner with a silicon micromirror is described in U.S. Pat. No. 4,317,611. FIGS. 1A and 1B are a plan view and a cross sectional view of the conventional example of the light beam scanner. Referring to FIGS. 1A and 1B, a mirror 2 formed of a silicon thin film is arranged in a concave section 1a which is provided for a supporting substrate 1. This mirror 2 is supported by the supporting substrate 1 through torsion bars 3a and 3b which are formed as a unit together with the mirror 2. Therefore, the mirror 2 can be vibrated by torsion of the torsion bars 3a and 3b. Also, the torsion bars 3a and 3b are formed of a conductive material. One end of each of the torsion bars 3a and 3b is electrically connected to a corresponding one of pads which are provided for the supporting substrate 1. Further, fixed electrodes 7a and 7b are supported by an insulating member 6 on both sides of the concave section 1a of the supporting substrate 1. These fixed electrodes 7a and 7b are respectively arranged above both sides of the mirror 2 to oppose to each other with a predetermined gap, e.g. 10 .mu.m to the a plane surface of the mirror 2.
In this light beam scanner, a high voltage of about 500 V is applied between the pad 8a of one of the fixed electrodes 7a and 7b and the pad 4a to connected to the torsion bar 3a, i.e., between the fixed electrode 7a and the mirror 2. As a result, electrostatic force is generated between the fixed electrode 7a and the mirror 2. One end of the mirror 2 is attracted to the side of the fixed electrode 7a by the attractive electrostatic force. The torsion bars 3a and 3b are twisted and deformed through the attracting operation to vibrate the mirror 2 in a counterclockwise direction as shown in the figure. Also, when the application of the high voltage to the fixed electrode 7a is stopped immediately after this a vibrating operation, the mirror 2 is vibrated in a direction opposite to the above direction because of torsion restoration force of the torsion bars 3a and 3b. When a high voltage is applied to the other fixed electrode 7b, the above vibrating operation is performed in the same manner. Therefore, by repeating the application and stop of the high voltage, the mirror 2 is made to vibrate. If a laser beam from a light source out of the figure is reflected by the mirror 2, the laser beam can be deflected and scanned.
In the conventional example of the light beam scanner, the vibration angle of the mirror 2 is limited to the angle position for each of ends of the mirror 2 to collide with the corresponding one of the fixed electrodes 7a and 7b, as shown by .theta.1 in FIG. 12. As mentioned above, because it is designed in such a manner that the gap distance between the mirror 2 and the fixed electrodes 7a and 7b is about 10 .mu.m, the vibration angle .theta.1 is generally about 1 degree in maximum, although depending on the size of the mirror. For this reason, the deflection angle when a laser beam is scanned becomes about 2 degrees from the law of reflection.
In such a small deflection angle, however, when a light beam scanner is applied to the above-mentioned barcode reader or laser printer, it is necessary to take a long light traveling length of the laser beam so that the light beam can be scanned in a required length region. This prevents the miniaturization of an optical apparatus.
Also, the resolution of the light beam obtained when the light beam is scanned by the mirror having a predetermined size is proportional to the deflection angle. Therefore, in order to obtain the light beam of the high resolution, it is desirable to make the maximum deflection angle of the light beam large.
For this purpose, it is considered that the gap distance between the mirror 2 and the fixed electrodes 7a and 7b is made large, so that the maximum vibration angle .theta.1 of the mirror 2 is increased, resulting in increase of the deflection angle.
However, a higher voltage is needed as the voltage applied between the mirror 2 and the fixed electrodes 7a and 7b with increase of this gap distance to generate necessary attractive electrostatic force. Thus, a power supply circuit is made large in size and another problem on the safety occurs.
A piezoelectric light beam scanner is described in Japanese Laid Open Utility Model Application (JP-U-Heisei 3-89413). In this reference, a piezoelectric element is elastically deformed to scan a light beams.
A torsion vibrator is described in Japanese Laid Open Patent Application (JP-A-Heisei 6-175060). In this reference, a mirror is vibrated between the fixed electrodes and is not vibrated beyond the fixed electrode. Therefore, a vibration angle is limited by the distance between the mirror and the fixed electrode.
A two-dimensional light beam scanner is described in Japanese Laid Open Patent Application (JP-A-Heisei 7-199099). In this reference, a mirror is driven by use of a piezoelectric element which is provided in a peripheral portion of the mirror.